Known apparatus for extracorporeal treatment of blood include at least one treatment unit (for example a dialyser or a filter, or an ultrafilter or a plasma filter or a filter unit of any other nature) having a semipermeable membrane which separates the unit of treatment into two chambers. An extracorporeal blood circuit allows the circulation of blood taken from a patient internally of the first chamber. At the same time, and typically in a countercurrent direction to the blood, a treatment fluid is circulated through a special circuit in the second chamber of the treatment unit. This type of equipment for blood treatment, known as dialysis apparatus, may be used for the removal of solutes and excess fluid from the blood of patients suffering from renal failure.
The extracorporeal blood circuit also includes two expansion chambers, also called bubble-traps, respectively located on a blood removal line from the patient and on a return blood line to the patient.
The expansion chambers, during the treatment, contain a predetermined quantity of blood up to a depth and a predetermined quantity of gas (air) in the remaining part of the chamber.
Clearly, for a safe operation of the extracorporeal treatment, the level of blood should never fall below a critical minimum level that could lead to the introduction of air into the extracorporeal circulation lines and subsequent potential infusion of the air into the circulatory system of the patient, with serious consequences.
Since the risks of such an event exist, and the problems caused to the patient are extremely serious, if not critical, the known dialysis machines are equipped with safety systems that may detect such an event and, should it occur, may place the patient in safety.
In particular, a device is in general provided on the return line blood to the patient, just before the vascular access and downstream of the venous expansion chamber, which device is directly connected to the unit control of the machine and is for the detection of air bubbles in the blood.
In the event that air is detected in the venous line, the control unit activates a patient safety procedure for the isolation of the patient by at least closing clamps on the extracorporeal blood circuit and shutting down the blood pump.
In addition to this safety device, some machines are also equipped with appropriate blood level sensors, optical or acoustic, in the venous expansion chamber (more rarely also in the arterial expansion chamber) able to signal the reaching of a minimum level that requires the intervention of specialized personnel to restore the correct quantity of blood in the chamber such as to avoid risks to the patient.
These systems, while fulfilling the tasks to which they are directed, incur additional costs and changes to the hardware of a machine on which they are or are to be installed.
Particularly because of the cost, these security systems are generally present only on the return line of the blood downstream of the dialyzer.
Furthermore, it is also worth mentioning that the bubble sensor device is generally able to reliably detect only bubbles of a certain size, while not possessing a sensitivity that would enable micro-bubbles of air dissolved in the blood to be detected.
There are recent studies (e.g. “Microemboli, developed during hemodialysis, pass the lung barrier and may cause ischemic lesions in organs such as the brain” by Ulf Forsberg, Per Jonsson, Christofer Stegmayr and Bernd Stegmayr) that have linked some typical disorders of chronic patients such as pulmonary hypertension and other ischemic problems with the quantity of air, in the form of micro-bubbles, generated by current dialysis machines and which are not disclosed by the current safety systems.
It should be noted in this regard that the generation of micro-bubbles occurs mainly because of the entry of air into the removal line, for example due to a low level of blood in the blood chamber (owing to various situations such as bad machine priming or infusion with air entry); in fact the bubbles that may get into the bloodstream and reach the dialyzer, which fragments them and makes them difficult to detect.
Also known from the U.S. Pat. No. 7,013,727 is a method for determining the blood level in a chamber of dialysis machines which exploits the ideal gas law in order to return to that level.
In particular, it exploits the change in blood volume in the chamber linked to the thrust generated on the blood by a peristaltic pump and, by means of two sensors (pressure and/or flow) the level in the chamber is detected.
This methodology, while enabling doing without a level sensor, typically requires additional hardware (a further sensor) in addition to that already present on the machine.